1. Field of the Invention
This invention relates generally to a receiver for a wireless telecommunications system and, more particularly, to a receiver for a wireless telecommunications system, where the receiver is a wideband receiver that can process multiple signal channels simultaneously.
2. Discussion of the Related Art
Wireless telecommunications systems, particularly cellular telephone communications systems, employ strategically placed base stations having transceivers that receive and transmit signals over a particular carrier frequency band to provide wireless communications between two parties. Depending on the particular area, each base station will include a certain number of receivers that receive the signals, and provide digital signal processing so that the signal is transmitted to the desired destination. Each receiver scans the frequency band of interest until it locks onto a signal in that band. The frequency band is usually at about 800 MHz and higher with a bandwidth of 200 KHz or more. An example with a bandwidth of 75 MHz centered at about 1750 MHz will be used in this description.
A typical receiver for this application will include an antenna that receives the signals and a duplexer that limits the received signals to the desirable carrier frequency range in which the signals are transmitted. The frequency band limited signal received by the antenna is then applied to a low noise amplifier (LNA) where it is amplified to an amplitude suitable for subsequent processing. The band limited and amplified signal is then applied to a frequency down-converter that mixes the received signal with a local oscillator (LO) signal to generate a lower intermediate frequency (IF) signal that can be readily processed by the DSP devices. A bandpass filter (BPF) is typically employed in the frequency down-converter to limit the IF signal to the particular frequency band of the received signal. The filtered IF signal is then applied to an analog-to-digital converter (ADC) that converts the IF signal to a representative digital signal that is then processed by the DSP devices. The receiver may be designed to process signals for many protocols and standards, including time division multiple access (TDMA), code division multiple access (CDMA), enhanced data rate for global evolution (EDGE), Gaussian minimal shift keying (GMSK), etc., depending on the particular application.
The known receiver described above for wireless telecommunications has only been capable of processing a single signal or a single channel at any given time. Thus, a separate receiver has been required in the base station for each separate signal being processed. Because certain base stations may have to receive and transmit many signals for multiple users simultaneously, these base stations must include enough receivers to satisfy this requirement, or service is lost. Because each receiver includes the various components that perform the signal receiving operation discussed above, high traffic base stations include a large amount of receiver hardware, and thus are costly. As cellular telephone communications increases, base stations will be required to provide more simultaneous access, or more base stations will be required, thus significantly increasing costs in these systems.
What is needed is a wireless telecommunications system receiver that operates over a relatively wide bandwidth, and is capable of processing multiple signals in different frequency bands simultaneously. To process multiple signals simultaneously, the basic components, including the LNA, the frequency down-converter and the ADC, of the receiver need to be redesigned to provide the performance specifications and requirements necessary. Compared to the single channel receiver previously mentioned, the multiple channel receiver must continuously handle a much wider range of frequencies and larger range of power levels. In addition, the multiple channel receiver must be insensitive to potential cross-channel interference. It is therefore an object of the present invention to provide such a receiver having high performance components.
In accordance with the teachings of the present invention, a receiver for a wireless telecommunications system is disclosed that provides relatively wideband signal processing of received signals without increased signal distortion so that multiple received signals can be simultaneously processed. The receiver includes a specialized LNA, frequency down-converter and ADC to perform the wideband signal processing while maintaining receiver performance. The frequency down-converter employs a suitable mixer, BPF, attenuator and transformer that are tuned to provide the desired frequency down-conversion and amplitude control over the desired wide bandwidth. The down-converter devices are selected depending on the particular performance criteria of the ADC. A specialized digital channelizer is included in the receiver that receives the digital signal from the ADC, and separates the signals into the multiple channels.
In one embodiment, the frequency down-conversion is performed in a single down-conversion process, and the ADC employs delta-sigma processing to provide the digital conversion over the complete frequency band. In an alternate embodiment, the frequency down-conversion is performed in a double down-conversion process so that a less complex ADC can be used.